The US Public Health Service initiated a research program in the 1950s on the health of US uranium miners employed in the Colorado Plateau. Occupational exposures in the early years of mine operation resulted in large excesses of mortality due to respiratory diseases that were readily-detected by classical epidemiological methods. These findings were highly influential for informing understanding of the carcinogenic effect of radon exposures in the US and abroad. Today, despite important reductions in occupational exposures, radon remains a leading cause of lung cancer, with potential for relatively high occupational exposures among workers in a wide range of settings including underground metal and non-metal mining, subway and utility tunnels, phosphate fertilizer plants, natural gas and oil piping facilities, oil refineries, and for people employed as radon remediation workers and water treatment workers. Other workplaces including schools, hospitals, and prisons also may have areas with high radon levels. To strengthen the basis for protection of contemporary workers from the carcinogenic effects of radon, and to improve compensation decisions for workers exposed in the past, we propose a pooled analysis of recently-updated cohorts of US underground miners. State-of-the-art statistical methods for longitudinal data analysis will be used to assess the effects of time-since- exposure, age-at-exposure, and attained age. In addition, we will examine the joint effects of radon and smoking in these cohorts, leveraging substantial gains in information on lung cancer among non-smokers provided by recent updates of these cohorts. Finally, because a causal interpretation of epidemiological findings is strengthened by evidence of reproducibility and consistency, we will assess the consistency of results derived from analysis of US underground miner cohorts with associations observed in other international cohorts by conducting a pooled analysis incorporating data from major cohorts of miners from Canada, the Czech Republic, Germany, France, and China. The findings of this research project, which will yield the most precise radon risk estimates reported to-date, are expected to have substantial impact on understanding of the effects occupational and environmental radon exposures.